A one-to-many relationship in a database is the most common relational database design and is at the heart of good design.
Example of a One-to-Many Relationship
Consider the relationship between a teacher and the courses they teach. A teacher can teach multiple classes, but the course would not have the same relationship with the teacher. Therefore, for each record in a Teachers table, there could be many records in the Courses table. This example illustrates a one-to-many relationship: one teacher to multiple courses.
Why Establishing a One-to-Many Relationship Is Important
To represent a one-to-many relationship, you need at least two tables. Let’s see why.
Adherence to First Normal Form Design
Perhaps we created a table in which we want to record the name and courses taught. We might design a Teachers and Courses table like this: This design also violates the first principle of database normalization, First Normal Form (1NF), which states that each table cell should contain a single, discrete piece of data.
The Second Normal Form Rule
Another design alternative might be to add a second record for Carmen: For example, what if Carmen’s name changed? Someone working with the data might update her name in one record and fail to update it in the second record. This design violates the Second Normal Form (2NF) standard, which adheres to 1NF and must also avoid the redundancies of multiple records. The 2NF rule achieves this by separating subsets of data into multiple tables and creating a relationship between them.
How to Design a Database With One-to-Many Relationships
To implement a one-to-many relationship in the Teachers and Courses table, break the tables into two and link them using a foreign key. Here, we removed the Course column in the Teachers table: We can see how this design avoids any possible redundancies, allows individual teachers to teach multiple courses, and implements a one-to-many relationship.
